Platelet adhesion is an initiating event in hemostasis. It is the mechanism by which the platelet recognizes foreign surfaces and attaches to those surfaces. This same process occurs when the subendothelial tissue is exposed following removal of the endothelial cell. The mediation of the platelet adhesion process is complex and involves the metabolic processes of the platelet, the plasma components, and the characteristics of the surface to which the platelet adheres. This project proposes to examine the adhesion of platelets to specific proteins which have been identified as important in this phenomenon. von Willebrand factor, collagen, and fibronectin will be purified and insolublized on polycarbonate, regenerated cellulose, agarose, and glass surfaces. These surfaces will then be exposed to moving platelets (whole blood, platelet rich plasma, or washed platelets) and the adhesion characteristics will be measured. Using the Richardson parallel plate laminar flow chamber and capillary tube aspiration model, it will be possible to perform experiments under precisely controlled conditions of shear stress. Time, protein concentration, and shear stress which are optimal for the reaction will be determined. The release platelet dense body and alpha granule release in response to these adhesion reactions will be measured. Experiments will also be performed to modify platelet metabolism and determine the effect of reducing thromboxane generation or increasing platelet cAMP on the adhesion reacton. Solid-phase proteins will be modified by partial reduction and alteration of the carbohydrate to determine the structural characteristics which mediate platelet adhesion to these proteins. In addition the platelet substrate-attached material, which is left by the platelet following detachment, will be characterized. The further understanding of the adhesion reaction will lead to greater understanding of atherogenesis, and hemostasie process. These reactions are also of importance in the interaction of the blood with synthetic materials placed in the circulation.